1966 Belvedere II... speedometer saga

[John Burns]

I'm working on my friend's 1966 and I can't get the speedometer to behave correctly... it bounces all over wildly. I've lubed the cable, cleaned the gears in the speedo, cleaned and re-oiled the wick, and it runs perfectly with a variable speed drill hooked up (and run backwards) at the transmission end. I've had 3 different driven gears in it- the teeth look perfect on all of them. The drive gears machined in the shaft (727) look fine. The only thing I haven't replaced is the speedometer gear housing. There isn't much to one of these- is there something going on with this that could cause this issue?
Thanks
John

 

[Sam69sat]

Just a thought but i wonder if there's problem with the cable near the transmission end where it curves and goes into the transmissioin? When you're running it with a drill you probably have the cable straight. Also heard it happening on the other end where it goes into the speedometer guage. Some have fixed it by trying to line up the cable as straight as possible where it goes into the guage.

 

[John Burns]

Just a thought but i wonder if there's problem with the cable near the transmission end where it curves and goes into the transmissioin? When you're running it with a drill you probably have the cable straight. Also heard it happening on the other end where it goes into the speedometer guage. Some have fixed it by trying to line up the cable as straight as possible where it goes into the guage.

I was just wondering the same thing. You are exactly right- I had the cable straight. I tried to simulate the curve but I couldn't see the damn speedo laying under the car with no one else around. Thank you very much for this thought. I will dig into it more.

 

[Jerry Hall]

I believe there is a bushings that if worn that lets this happen. The last 2 1967 GTX 150 mph speedometers I had were sent off for the exact same thing. I don't know where they were sent as the guys that restored the cluster sent them out.

 

[Don Frelier]

If you can pull the core out of the sheath then hold one end in each hand leaving a u shaped loop towards the floor. Spin the cable with one hand and let the other just rest loosely inside the other. If it bounces around you have broken strands in the core.

 

[YY1]

Didn't see mentioned the adjustment to the gear mesh depth by turning the tail shaft end housing.

 

[John Burns]

Didn't see mentioned the adjustment to the gear mesh depth by turning the tail shaft end housing.

I'm not aware of this trick- what is involved?

 

[YY1]

The cable goes in the housing off-center.

You adjust the gear contact depth by turning it.

It does not necessarily need to end up where the clamp rests in the grooves.

 

[AR67GTX]

I had that issue with my 67 GTX once. It was rebuilt by one of the big specialist and when I installed it, it jumped around. Played around with the transmission mesh adjustment - jumped. Tried driving it with a drill - jumped. Lubed up the cable and it jumped. Pulled it back out and drove it through a cable on bench with new cable - jumped. Shipped it back to company, and they said it behaved perfect for them. When I got it back, tried it on the bench again and jumped. Deciding I had lived with it doing this for 15 years so I gave up and put it back in the car with the new cable - works dead smooth and steady. No idea what the hell the issue was. You have my deepest sympathy and understanding.

 

[John Burns]

The cable goes in the housing off-center.

You adjust the gear contact depth by turning it.

It does not necessarily need to end up where the clamp rests in the grooves.

Interesting- thanks for the idea! I've had 3 different speedo gears- each one on the lower end of the clock ranges (26, 33, 40... long story). They have all behaved the same way. Maybe clocking will do the trick.

 

[John Burns]

One other note- and I should have mentioned this in my OP... the "correct" gear is supposed to be the 33 tooth according to the chart based on tires and gear (28" and 3.55). The speedometer ran very linearly 10 mph fast. Some calculation showed that it needed the 40 tooth to correct for the 10 mph. This doesn't really make any sense... The car is a 66 (no idea what year the trans is), but I know things changed between 65 and 66 regarding this stuff... do I have some weird mismatch of parts?

 

[YY1]

Adjusting that makes up for the difference in size for different tooth count gears.

 

[joe smith]

I had problems with my 67 until I had Shannon at Redline build me a cable and it has been perfect since... JUNK cables being sold

 

[John Burns]

CURED!! A new cable fixed it. What is so weird to me is that it ran fine hooked to a drill. I pulled the cable from the housing and it didn't wobble or anything. But when I pulled the whole thing out, the cable cover was a bit of a pretzel.

And one more weird thing- the speedo was stable with the 33 tooth gear, but it was 10 mph too slow. The 33 tooth is "correct" per the gear / tire / gear chart. Then I tossed in the 40 tooth - and it started out jumping around. But I just drove it for about an hour and it has bedded in... nice and stable and it is accurate!

What a saga- I appreciate everyone's thoughts on this.

(One note from eBayer "mcguyver!" for some things to look for if you are ever in the speedometer pain barrel: Can’t be the housing. Could be the cable. Look for damage on the exterior, burning from exhaust, if none, try loosely attaching the cable nut to the adapter . Run it, See if it bounces. Try with both gears. Maybe there’s a difference between the gears, or a slightly different cable or how much the cable engages in the gear itself. , or how much the cable engages to the speedometer head. If the gear forces the cable slightly too much toward the speedometer head, might be binding causing jumping. This is weird stuff.)